The most important part of SEO is building great links to your content. The most common mistake is targeting the wrong keywords. The question at hand is, how do I choose my keywords? I have a great suggestion that should help you out.
Google trends is a great help with choosing what you should target. Google trends will show you how much traffic a search term in Google is getting. The power is not in knowing how much traffic it gets, but how much traffic it gets relative to a similar term. Because you get no hard numbers you must remember to keep everything relative. I like to do a common search with every query so I know how much traffic I get relative to the keyword I learn about.
For example, if I want to write an article for an article website, I may try to test Free Content, and Free Articles. To compare these two searches I separate them with a comma and hit search. Then I see that Free Articles is very close to Free Content. I can also choose to add a third keyword if I would like.
I used this tool to optimize many of my pages. By targeting keywords that are searched more you have a lot more people to work for.